Geochemistry Of Sediments From Changjiang Estuary:Implications For Provenance And Chemical Weathering

In this study,the radioisotope dating,grain size and multi-element geochemical data of sediment core CM-A from Changjiang estuary were analyzed in order to determine CM-A's chemical composition,provenance,sedimentary sorting and intensity of chemical weathering.We also try to understand the geological meaning of the CM-A sediments in a large frame of river sediments in Chinese mainland by comparing the CM-A data with the sample data of Yellow river and Pearl river.Data of radioisotope dating indicate that the age of deposition of CM-A core is 1973²010.The main components of CM-A sediments are silts?grain size <63?m?according to Fock classification diagram,therefore the CM-A core is perfectly adequate for provenance research using REEs.The contents of major oxides such as Al₂O₃?Fe₂O₃?K₂O?TiO₂ in CM-A sediments are higher than that of Yellow river sample and lower than that of Pearl river sample,which might be controlled mainly by climate and chemical weathering of drainage basin.The contents of many trace elements such as Cr?Li?Sc?Cu?Zn?Y?Nb in CM-A sediments are high compared with Yellow river sample and relatively low compared with Pearl river sample,which might be affected mainly by source rock type and chemical weathering of river basin.Distributions of most major and trace elements in the depth profile are dominated by a double-dip pattern,according to which the contents of these elements are very low during 1988²002 and 2002²005,which might be affected mainly by human activities.REE patterns indicate that CM-A sediments are similar with the sample of Yellow river and Pearl river,which may imply that all of them have a terrigenous source.REE patterns suggest that CM-A sediments are similar with SPM of upstream of Changjiang,which may imply that the upstream of Chanjiang is the main source area of CM-A sediments,which is accordant with previous studies.A-CN-K diagram indicates low to moderate chemical weathering for CM-A sediments,and the CIA value is higher than that of Yellow river sample and lower than that of Pearl river sample,which might be controlled mainly by climate conditions in drainage area.CIA value of CM-A is low compared with average shales,which probablely means that the extent of chemical weathering in Changjiang drainage basin is relatively small compared with world level.This diagram also suggests that the source rock composition of CM-A sediments is similar with granodiorite.SiO_2-?A1₂O₃ + K₂O +Na₂O?diagram demonstrates that the grade of chemical maturity of CM-A sediments is higher than that of Pearl river sample and average shales,which may imply that sediments from Changjiang estuary have a relatively long-time-transporting history.Zr/Sc-Th/Sc diagram indicates that the extent of sedimentary sorting of CM-A sediments is smaller than that of Yellow river sample.